ray Interactions with Matter

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Radiography
x-ray
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RADIATION THERAPY
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X-rays
Röntgen realized the importance of his discovery,
and communicated this quickly. He did not patent
his discovery, nor did he charge royalties. His
magnanimity meant that the capability to make
roentgenograms spread incredibly quickly
worldwide. On the North American continent,the
first diagnostic roentgenogram (of a boy with a
Colles fracture) was made at Dartmouth College,
Hanover, New Hampshire, on February 3, 1896, just
2 months after Röntgens paper on the discovery
was published
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X-rays
On November the 8th 1895, Wilhelm Conrad Röntgen,
Professor für Physik in Würzburg, Germany,
discovered a "new kind of rays, from the effect
they had on photographic film. He called them
x-rays. He received the first Nobel Prize in
1901 for his discovery because he did not just
discover them, but made a systematic
investigation of their properties . He presented
data on the effects of thickness, density and
atomic number on the attenuation properties -
just two months after his initial discovery.
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X-rays
Some of the earliest x-ray pictures were taken of
hands. Röntgen x-rayed his own hand and the the
hand of his wife. This picture was taken from
the Röntgen museum.
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Early Use of Contrast Agents
The use of contrast agents was also developed
incredibly quickly (even if not in vivo). A
radio-opaque dye was injected into the hand of a
corpse (an elderly woman) and the vein and artery
structure in the hand imaged. This x-radiograph
was taken months after the initial discovery.
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Principle of Radiography
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Radiographs Record Transmitted Intensity
For x and gamma-radiation, photons are attenuated
by material IIoe-(?/?)?x where- Io is the
initial intensity, x is the distance travelled
through the material (?/?) is the mass
attenuation coefficient for the material ? is the
density of the material
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X-radiography In Vivo

• There is more x-ray attenuation with-
• increasing mass attenuation coefficients (?/?)
• increasing density
• x-radiography is therefore useful for imaging the
  human body because
• calcified tissues such as bones and teeth are
  denser than soft tissue
• calcified tissues have a higher mass attenuation
  coefficients than soft tissue

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X-radiographs
A broken arm
A broken leg
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Dental X-rays
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Mammography
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?? and x-ray Interactions with Matter
There are 3 dominant interaction mechanisms of ?
and x-rays with matter. These interactions are
the means by which ? and x-rays are attenuated
through material. Photoelectric
Absorption Compton Scattering Pair Production
The predominant mode of interaction depends on
the energy of the incident photons and the atomic
number of the material.
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Photoelectric Effect
Above the energy threshold of the absorption
edge, the incident photon is completely
absorbed. A photoelectron is ejected from the
electron shell with energy Ee h? - Eb The
vacancy in the shell is filled by an electron
from an outer shell with the subsequent emission
of either - characteristic x-rays - Auger
electrons
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Compton Scattering
A photon is scattered off of an electron. The
electron recoils. The scattered photon is reduced
in energy. The energy of the scattered photon
depends on the angle ? through which it is
scattered
?
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Pair Production
Above the threshold of 1.022 MeV (twice the
electron rest mass) pair production can take
place.
In the field of the nucleus, an electron positron
pair can be created.
e -
incident ?-ray
If the energy of the ?-ray is greater than 1.022
MeV, then the excess energy is given to the
positron/electron pair as kinetic energy. This
kinetic energy is deposited in the detector
volume.
e
511 keV
annihilation
511 keV
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Pair Production
The positron slows in the detector, until it is
nearly stationary. It forms a short-lived state,
analagous to hydrogen with an electron, called
positronium. In over 95 of decays, the electron
and the positron annihilate with the emission of
two 511 keV photons at 180 degrees to each other.
e -
incident ?-ray
e
511 keV
annihilation
511 keV